This invention was made with government support under Contract No. 38-83CH10093 awarded by DOE. The government may have certain rights in the invention.
Capacitors are storage devices that store electrical energy on an electrode surface. Electrochemical cells create an electrical charge at electrodes by chemical reaction. The ability to store or create electrical charge is a function of electrode surface area in both applications. Ultracapacitors, sometimes referred to as double layer capacitors, are a third type of storage device. An ultracapacitor creates and stores energy by microscopic charge separation at an electrical chemical interface between electrode and electrolyte.
Ultracapacitors are able to store more energy per weight than traditional capacitors and they typically deliver the energy at a higher power rating than many rechargeable batteries. Ultracapacitors comprise two porous electrodes that are isolated from electrical contact by a porous separator. The separator and the electrodes are impregnated with an electrolytic solution, which allows ionic current to flow between the electrodes while preventing electronic current from discharging the cell. On the back of each electrode is a current collector. One purpose of the current collector is to reduce ohmic loss. If the current collectors are nonporous, they can also be used as part of the capacitor case and seal.
When electric potential is applied to an ultracapacitor cell, ionic current flows due to the attraction of anions to the positive electrode and cations to the negative electrode. Upon reaching the electrode surface, the ionic charge accumulates to create a layer at the solid liquid interface region. This is accomplished by absorption of the charge species themselves and by realignment of dipoles of the solvent molecule. The absorbed charge is held in this region by opposite charges in the solid electrode to generate an electrode potential. This potential increases in a generally linear fashion with the quantity of charge species or ions stored on the electrode surfaces. During discharge, the electrode potential or voltage that exists across the ultracapacitor electrodes causes ionic current to flow as anions are discharged from the surface of the positive electrode and cations are discharged from the surface of the negative electrode while an electronic current flows through an external circuit between electrode current collectors.
In summary, the ultracapacitor stores energy by separation of positive and negative charges at the interface between electrode and electrolyte. An electrical double layer at this location consists of sorbed ions on the electrode as well as solvated ions. Proximity between the electrodes and solvated ions is limited by a separation sheath to create positive and negative charges separated by a distance which produces a true capacitance in the electrical sense.
During use, an ultracapacitor cell is discharged by connecting the electrical connectors to an electrical device such as a portable radio, an electric motor, light emitting diode or other electrical device. The ultracapacitor is not a primary cell but can be recharged. The process of charging and discharging may be repeated over and over. For example, after discharging an ultracapacitor by powering an electrical device, the ultracapacitor can be recharged by supplying potential to the connectors.
The physical processes involved in energy storage in an ultracapacitor are distinctly different from the electrochemical oxidation/reduction processes responsible for charge storage in batteries. Further unlike parallel plate capacitors, ultracapacitors store charge at an atomic level between electrode and electrolyte. The double layer charge storage mechanism of an ultracapacitor is highly efficient and can produce high specific capacitance, up to several hundred Farads per cubic centimeter.
Ultracapacitors are multilayer structures that include two solid, nonporous current collectors, two porous electrodes separating the collectors and a porous separator between the electrodes. A nonaqueous electrolyte solution saturates the electrodes and separator layer. The electrolyte solution includes an organic solvent and an electrolyte. The structure is sealed to form the multilayer ultracapacitor. The electrolyte solution presents a deleterious environment that adversely affects sealants that are used to close and seal the layers of the ultracapacitor. The dielectric constant of the ultracapacitor depends upon the proportion of electrolyte salt to solute. The electrolyte solution breaks down sealant and causes loss of electrolyte through evaporation to change the proportion of electrolyte to solute. The present invention relates to a sealant and method of sealing an ultracapacitor that eliminates chemical interaction and mechanical degradation to an ultracapacitor seal that is caused by degradation of sealant by electrolyte solution. According to the present invention, the ultracapacitor conductor layer is sealed to an electrode by means of a vinyl acetate polymer or polyamide fusible binder. These polymeric fusible binders withstand electrolyte chemical attack to maintain ultracapacitor integrity, flexibility and barrier properties.
The invention relates to a method of making an ultracapacitor. In the method, a cell is provided that comprises two solid, nonporous current collectors, two porous electrodes separating the current collectors, a porous separator between the electrodes and an electrolyte occupying pores in the electrodes and separator. A thermoplastic vinyl acetate polymer or thermoplastic polyamide is applied to the multilayer structure; and pressure or heat is applied to seal layers of the cell by means of the thermoplastic vinyl acetate polymer or thermoplastic polyamide to form the ultracapacitor.
The invention also relates to an ultracapacitor that is sealed by means of the thermoplastic vinyl acetate polymer or thermoplastic polyamide.